# Copyright (c) 2000-2002 Proofpoint, Inc. and its suppliers. # All rights reserved. # # By using this file, you agree to the terms and conditions set # forth in the LICENSE file which can be found at the top level of # the sendmail distribution. # # $Id: SECURITY,v 1.52 2013-11-22 20:51:54 ca Exp $ # This file gives some hints how to configure and run sendmail for people who are very security conscious (you should be...). Even though sendmail goes through great lengths to assure that it can't be compromised even if the system it is running on is incorrectly or insecurely configured, it can't work around everything. This has been demonstrated by recent OS problems which have subsequently been used to compromise the root account using sendmail as a vector. One way to minimize the possibility of such problems is to install sendmail without set-user-ID root, which avoids local exploits. This configuration, which is the default starting with 8.12, is described in the first section of this security guide. ***************************************************** ** sendmail configuration without set-user-ID root ** ***************************************************** sendmail needs to run as root for several purposes: - bind to port 25 - call the local delivery agent (LDA) as root (or other user) if the LDA isn't set-user-ID root (unless some other method of storing e-mail in local mailboxes is used). - read .forward files - write e-mail submitted via the command line to the queue directory. Only the last item requires a set-user-ID/set-group-ID program to avoid problems with a world-writable directory. It is however sufficient to have a set-group-ID program and a group-writable queue directory. The other requirements listed above can be fulfilled by a sendmail daemon that is started by root. Hence this section explains how to use two sendmail configurations to accomplish the goal to have a sendmail binary that is not set-user-ID root, and hence is not open to system configuration/OS problems or at least less problematic in presence of those. The default configuration starting with sendmail 8.12 uses one sendmail binary which acts differently based on operation mode and supplied options. sendmail must be a set-group-ID (default group: smmsp, recommended gid: 25) program to allow for queueing mail in a group-writable directory. Two .cf files are required: sendmail.cf for the daemon and submit.cf for the submission program. The following permissions should be used: -r-xr-sr-x root smmsp ... /PATH/TO/sendmail drwxrwx--- smmsp smmsp ... /var/spool/clientmqueue drwx------ root wheel ... /var/spool/mqueue -r--r--r-- root wheel ... /etc/mail/sendmail.cf -r--r--r-- root wheel ... /etc/mail/submit.cf [Notice: On some OS "wheel" is not used but "bin" or "root" instead, however, this is not important here.] That is, the owner of sendmail is root, the group is smmsp, and the binary is set-group-ID. The client mail queue is owned by smmsp with group smmsp and is group writable. The client mail queue directory must be writable by smmsp, but it must not be accessible for others. That is, do not use world read or execute permissions. In submit.cf the option UseMSP must be set, and QueueFileMode must be set to 0660. submit.cf is available in cf/cf/, which has been built from cf/cf/submit.mc. The file can be used as-is, if you want to add more options, use cf/cf/submit.mc as starting point and read cf/README: MESSAGE SUBMISSION PROGRAM carefully. The .cf file is chosen based on the operation mode. For -bm (default), -bs, and -t it is submit.cf (if it exists) for all others it is sendmail.cf. This selection can be changed by -Ac or -Am (alternative .cf file: client or mta). The daemon must be started by root as usual, e.g., /PATH/TO/sendmail -L sm-mta -bd -q1h (replace /PATH/TO with the right path for your OS, e.g., /usr/sbin or /usr/lib). Notice: if you run sendmail from inetd (which in general is not a good idea), you must specify -Am in addition to -bs. Mail will end up in the client queue if the daemon doesn't accept connections or if an address is temporarily not resolvable. The latter problem can be minimized by using FEATURE(`nocanonify', `canonify_hosts') define(`confDIRECT_SUBMISSION_MODIFIERS', `C') which, however, may have undesired side effects. See cf/README for a discussion. In general it is necessary to clean the queue either via a cronjob or by running a daemon, e.g., /PATH/TO/sendmail -L sm-msp-queue -Ac -q30m If the option UseMSP is not set, sendmail will complain during queue runs about bogus file permission. If you want a queue runner for the client queue, you probably have to change OS specific scripts to accomplish this (check the man pages of your OS for more information.) You can start this program as root, it will change its user id to RunAsUser (smmsp by default, recommended uid: 25). This way smmsp does not need a valid shell. Summary ------- This is a brief summary how the two configuration files are used: sendmail.cf For the MTA (mail transmission agent) The MTA is started by root as daemon: /PATH/TO/sendmail -L sm-mta -bd -q1h it accepts SMTP connections (on ports 25 and 587 by default); it runs the main queue (/var/spool/mqueue by default). submit.cf For the MSP (mail submission program) The MSP is used to submit e-mails, hence it is invoked by programs (and maybe users); it does not run as SMTP daemon; it uses /var/spool/clientmqueue by default; it can be started to run that queue periodically: /PATH/TO/sendmail -L sm-msp-queue -Ac -q30m Hints and Troubleshooting ------------------------- RunAsUser: FEATURE(`msp') sets the option RunAsUser to smmsp. This user must have the group smmsp, i.e., the same group as the clientmqueue directory. If you specify a user whose primary group is not the same as that of the clientmqueue directory, then you should explicitly set the group, e.g., FEATURE(`msp') define(`confRUN_AS_USER', `mailmsp:smmsp') STARTTLS: If sendmail is compiled with STARTTLS support on a platform that does not have HASURANDOMDEV defined, you either need to specify the RandFile option (as for the MTA), or you have to turn off STARTTLS in the MSP, e.g., DAEMON_OPTIONS(`Name=NoMTA, Addr=127.0.0.1, M=S') FEATURE(`msp') CLIENT_OPTIONS(`Family=inet, Address=0.0.0.0, M=S') The first option is used to turn off STARTTLS when the MSP is invoked with -bs as some MUAs do. What doesn't work anymore ------------------------- Normal users can't use mailq anymore to see the MTA mail queue. There are several ways around it, e.g., changing QueueFileMode or giving users access via a program like sudo. sendmail -bv may give misleading output for normal users since it may not be able to access certain files, e.g., .forward files of other users. Alternative ----------- Instead of having one set-group-ID binary, it is possible to use two with different permissions: one for message submission (set-group-ID), one acting as daemon etc, which is only executable by root. In that case it is possible to remove features from the message submission program to have a smaller binary. You can use sh ./Build install-sm-mta to install a sendmail program to act as daemon etc under the name sm-mta. Set-User-Id ----------- If you really have to install sendmail set-user-ID root, first build the sendmail package normally using sh ./Build Then you can use sh ./Build install-set-user-id to install the package in the old (pre-8.12) way. Make sure that no submit.cf file is installed. See devtools/README about confSETUSERID_INSTALL which you need to define.